CURRENTS OF THE SEA. 149 



398. The fauna and the flora of the sea are as much the crea- 

 tures of climate (§ 76), and are as dependent for their well-being 

 upon temperature as are the fauna and the flora of the dry land. 

 Were it not so, we should find the fish and the alga?, the marine 

 insect and the coral, distributed equally and alike in all parts of 

 the ocean. The polar whale would delight in the torrid zone, and 

 the habitat of the pearl oyster would be also under the iceberg, or 

 in frigid waters colder than the melting ice. 



399. Now water, while its capacities for heat are scarcely ex- 

 ceeded by those of any other substance, is one of the most com- 

 plete of non-conductors. Heat does not permeate water as it does 

 iron, for instance, or other good conductors. Heat the top of an 

 iron plate, and the bottom becomes warm ; but heat the top of a 

 sheet of water, as in a pool or basin, and that at the bottom re- 

 mains cool. The heat passes through iron by conduction, but to 

 get through water it requires to be conveyed by a motion, which 

 in fluids we call currents. 



400. Therefore the study of the climates of the sea involves a • 

 knowledge of its currents, both cold and warm. They are the 

 channels through which the waters circulate, and by means of 

 which the harmonies of old ocean are preserved. 



401. Hence, in studying the system of oceanic circulation, we 

 set out with the very simple assumption, viz., that from whatever 

 part of the ocean a current is found to run, to the same part a 

 current of equal volume is bound to return ; for upon this princi- 

 ple is based the whole system of currents and counter-currents of 

 the air as well as of the water. 



402. Currents of water, like currents of air, meeting from vari- 

 ous directions, create gyrations, which in some parts of the sea, 

 as on the coast of Norway, assume the appearance of whirlpools, 

 as though the water were drawn into a chasm below. The cele- 

 brated Maelstrom is caused by such a conflict of tidal or other 

 streams. Admiral Beechey, R.N.,* has given diagrams illustrative 

 of many "rotatory streams in the English Channel, a number of 

 which occur between the outer extremities of the channel tide and 



* See an interesting paper by him on Tidal Streams of thxc North Sea and English 

 Channel, pp. 703 ; Phil. Transactions, Part ii., 1851. 



